Linear actuator

ABSTRACT

A linear actuator is provided with an outer tube and an inner tube inserted in the outer tube so as to be free to slide. The linear actuator is also provided with a rod erected at a central axis part of the inner tube and forming an annular space between the rod and the inner tube, a rod guide provided at an extreme end of the rod in sliding contact with an inner surface of the outer tube, a plurality of permanent magnets arranged along an axial direction in the rod, and a plurality of coils retained by the outer tube so as to face the permanent magnets. A cross-sectional area of an outer periphery of the inner tube along which the outer tube slides and a cross-sectional area of an inner periphery of the outer tube along which the rod guide slides are set to be equal.

TECHNICAL FIELD

This invention relates to a linear actuator.

BACKGROUND ART

JP2007-274820A published by the Japan Patent Office proposes a linearactuator provided with a cylindrical tube having a closed end, acylindrical yoke retaining permanent magnets arranged along an axialdirection on an outer peripheral surface, and a plurality of coilsdisposed on an inner surface of the tube. This linear actuator isdesigned to drive the yoke in the axial direction relative to the tube.

The linear actuator of JP2007-274820A is also provided with a rod whichis disposed in a bottom portion of the tube and can be moved forwardinto and backward from the yoke, a bearing disposed at an extreme end ofthe rod in sliding contact with an inner surface of the yoke, and abearing disposed on the inner surface of the yoke in sliding contactwith an outer peripheral surface of the rod. With these two bearingsserving to guide the moving yoke, the linear actuator is configured toextend and contract smoothly.

At a proximal end of the yoke, there is provided a cylindricalprotective cover having a closed end, the protective cover covering anopen end of the tube. On an inner surface of an open end portion of theprotective cover, there is provided a dust seal which slides along anouter peripheral surface of the tube. This dust seal prevents suchforeign matter as dust, dirt and drops of water from entering aninternal space of the linear actuator. In the linear actuator ofJP2007-274820A, the internal space thereof is sealed by the dust seal asmentioned above.

SUMMARY OF INVENTION

In the linear actuator of JP2007-274820A, an internal volumetriccapacity thereof varies as a result of extension and contraction.Particularly, when the internal volumetric capacity increases due toextension, an internal pressure of the linear actuator becomes lowrelative to an external pressure thereof, and a gap may be formedbetween the dust seal and the tube. This gap may allow foreign matter tointrude into the internal space.

It is therefore an object of this invention to prevent intrusion offoreign matter into the internal space of the linear actuator.

To achieve the object described above, a linear actuator includes anouter tube and an inner tube inserted in the outer tube, the linearactuator generating a thrust for relatively displacing the outer tubeand the inner tube along an axial direction. The linear actuator furtherincludes a rod erected at a central axis part of the inner tube andforming an annular space between the rod and the inner tube a rod guideprovided at an extreme end of the rod in sliding contact with an innersurface of the outer tube, a plurality of permanent magnets arrangedalong the axial direction in the rod, and a plurality of coils retainedby the outer tube so as to face the permanent magnets. A cross-sectionalarea of an outer periphery of the inner tube along which the outer tubeslides and a cross-sectional area of an inner periphery of the outertube along which the rod guide slides are set to be equal.

The details as well as other features and advantages of this inventionare set forth in the remainder of the specification and are shown in theaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a longitudinal cross-sectional view of a linear actuatoraccording to an embodiment of this invention; and

FIG. 2 is a longitudinal cross-sectional view of the linear actuatoraccording to the embodiment of this invention showing a state in whichthe linear actuator is most extended and a state in which the linearactuator is most contracted.

DESCRIPTION OF EMBODIMENTS

A linear actuator 1 according to an embodiment of this invention is willbe described with reference to the drawings.

Referring to FIG. 1 and FIG. 2, the configuration of the linear actuator1 will be described.

The linear actuator 1 is provided with a cylindrical outer tube 2, aninner tube 3 formed into a cylindrical shape having a closed end and anopen end portion which can be displaced axially in the outer tube 2, arod 4 having a proximal end joined to a bottom portion 3 a of the innertube 3 and an extreme end projecting from the inner tube 3, and a rodguide 5 provided at the extreme end of the rod 4 in sliding contact withan inner surface of the outer tube 2. The linear actuator 1 is alsoprovided with a magnetic field generator 6 having a plurality ofpermanent magnets 6 a retained on the inside of the rod 4, a pluralityof coils 7 which are so arranged as to face an outer peripheral surfaceof the magnetic field generator 6, and a cylindrical coil holder 15retaining the coils 7 on an inner surface, an outer peripheral surfaceof the coil holder 15 being fixed to the outer tube 2. The linearactuator 1 is a device which generates a thrust for relativelydisplacing the outer tube 2 and the inner tube 3 along the axialdirection according to a magnetic force that is generated between themagnetic field generator 6 and the coils 7 by exciting the coils 7.

In the following description of the outer tube 2 and its relatedelements, an open end into which the inner tube 3 is inserted isreferred to as an extreme end and an opposite end is referred to as aproximal end. Similarly, in the description of the inner tube 3 and itsrelated elements, an open end inserted into the outer tube 2 is referredto as an extreme end and an opposite end is referred to as a proximalend.

Respective elements of the linear actuator 1 will be described in detailbelow.

The outer tube 2 has a cylindrical base portion 11 and a cylindricalinner portion 12 which is joined to the base portion 11.

The base portion 11 is formed in such a way that an inner surface of aproximal end of the base portion 11 along which the rod guide 5 slideshas a smaller diameter as compared to an inner surface of an extreme endinto which the inner portion 12 is inserted. The base portion 11 has aguide sliding part 11 a in which the rod guide 5 is inserted so as to befree to slide and a fitting part 11 b connected to the guide slidingpart 11 a in an axial direction thereof, the fitting part 11 b having alarger diameter than the guide sliding part 11 a. A screw part 11 c isformed on an inner surface of an extreme end of the fitting part 11 b.The inner portion 12 is screwed into the screw part 11 c.

The inner portion 12 is an element which is screwed into the extreme endof the fitting part 11 b, the inner portion 12 having an inner surfacealong which the inner tube 3 slides. The inner portion 12 has a bearingfitting part 12 b formed on an inner surface of an extreme end portionthereof in which the inner tube 3 is inserted and a seal fitting part 12c. The seal fitting part 12 c is formed on an extreme end side of thebearing fitting part 12 b. A bearing 13 serving as an annular firstbearing which slides along an outer peripheral surface of the inner tube3 is provided in the bearing fitting part 12 b. An annular dynamic seal14 which slides along the outer peripheral surface of the inner tube 3is provided in the seal fitting part 12 c.

At a proximal end of the inner portion 12, there is formed a screw part12 a which is screwed into the screw part 11 c on the inner surface ofthe base portion 11. The inner portion 12 is joined integrally with thebase portion 11 as the screw part 12 a is screwed into the screw part 11c.

The plurality of coils 7 which are arranged along the axial directionare retained on an inner surface of the coil holder 15. The coils 7 areheld by the outer tube 2 via the coil holder 15. The coils 7 are sodisposed as to surround an outer peripheral surface of the rod 4 andface the permanent magnets 6 a provided in the rod 4. A total of sixcoils 7 are provided, two of each serving for U, V and W phases. Thecoils 7 are arranged in the order of the U, V and W phases.

Phase of current supplied to each phase of the coils 7 is switched onthe basis of electrical angles with respect to the magnetic fieldgenerator 6 and the magnitudes of currents are regulated by pulse widthmodulation (PWM) control, whereby the magnitude of the thrust generatedby the linear actuator 1 and a direction of the thrust are controlled.The number of the coils 7 is set to a value suited to the thrustgenerated by the linear actuator 1 and a pattern of electrical currentsupply.

Also, when an external force is exerted on the linear actuator 1 whichcauses the outer tube 2 and the inner tube 3 to be relatively displacedalong the axial direction, there is generated a force that damps therelative displacement caused by the external force as a result ofenergizing current supply to the coils 7 or an induced electromotiveforce generated in the coils 7. This serves to damp vibrations ordisplacement of various devices caused by the external force exerted onthe linear actuator 1.

The coil holder 15 has a cylindrical coil holding portion 15 a retainingthe coils 7 within an inner surface, a flange portion 15 b joineddirectly to a proximal end of the coil holding portion 15 a along theaxial direction, and a conducting hole 15 c passing through the flangeportion 15 b along the axial direction.

The coil holding portion 15 a is inserted in an annular space 21 formedbetween the rod 4 and the inner tube 3.

Provided within the flange portion 15 b is a magnetic sensor 16 whichserves as a position sensor for detecting the position of the rod 4along a stroke thereof. The magnetic sensor 16 detects the position ofthe magnetic field generator 6 relative to the coils 7 by sensingmagnetism of the permanent magnets 6 a.

The magnetic sensor 16 and the coils 7 are assembled in the coil holder15, together forming a cartridge-like structure. This makes it possibleto easily incorporate the magnetic sensor 16 and the coils 7 in theouter tube 2.

The flange portion 15 b is sandwiched between the base portion 11 andthe inner portion 12 and thereby fixed to the outer tube 2.Specifically, the flange portion 15 b is first fitted in the fittingpart 11 b of the base portion 11. The screw part 12 a of the innerportion 12 is then screwed into the screw part 11 c of the base portion11. As a result, the flange portion 15 b is sandwiched between a steppedstage 11 d of the base portion 11 and an extreme end 12 d of the innerportion 12.

The inner tube 3 has an extreme end that is inserted into the outer tube2 and the inner tube 3 is thereby maintained in state where it can moveforward into and backward from the outer tube 2. An opening is formed atthe extreme end of the inner tube 3. Specifically, the inner tube 3 isinserted into an annular space formed between the inner portion 12 ofthe outer tube 2 and the coil holder 15.

There is formed a bearing fitting part 3 c on the outer peripheralsurface of the extreme end of the inner tube 3. A bearing 17 serving asa second bearing which slides along the inner surface of the outer tube2 is provided in the bearing fitting part 3 c. The inner tube 3 isinserted into the outer tube 2 and supported by the bearing 13 and thebearing 17 such that the inner tube 3 can freely slide with respect tothe outer tube 2. A result of this arrangement is that a bending momentcaused by a lateral force exerted on the linear actuator 1 is supportedby the outer tube 2 and the inner tube 3.

There is formed a conducting hole 3 b interconnecting an air gap 20formed between the inner tube 3 and the outer tube 2 and an internalspace of the inner tube 3 on a side of the inner tube 3 in the vicinityof the bearing 17. The provision of the conducting hole 3 b prevents theair gap 20 from being completely closed off. This arrangement serves toprevent such a situation that a smooth extending or contracting actionof the linear actuator 1 is hindered by variations in pressure in theair gap 20 which may occur during extension or contraction of the linearactuator 1.

The proximal end of the rod 4 is joined to the bottom portion 3 a of theinner tube 3 at a proximal end thereof. The extreme end of the rod 4projects out from the opening in the extreme end of the inner tube 3 andreaches the guide sliding part 11 a of the base portion 11 of the outertube 2.

The rod 4 is erected at a central axis part of the inner tube 3, therebyforming the annular space 21 between the rod 4 the and inner tube 3. Therod 4 comprises a cylinder portion 4 a accommodating the permanentmagnets 6 a in an internal space, a plug 4 b which closes one open endof the cylinder portion 4 a and a plug 4 c which closes the other openend of the cylinder portion 4 a. The plug 4 b is joined to the bottomportion 3 a of the inner tube 3 while the plug 4 c is joined to the rodguide 5.

The plurality of rod-shaped permanent magnets 6 a arranged along theaxial direction are accommodated within the cylinder portion 4 a. As thepermanent magnets 6 a are thus accommodated in the cylinder portion 4 a,the permanent magnets 6 a are retained by the rod 4. In this embodiment,the permanent magnets 6 a are magnetized such that north (N) and south(S) poles are located along the axial direction. Any adjacent ones ofthe permanent magnets 6 a accommodated within the rod 4 are arrangedalong the axial direction in such a manner that magnetic poles of thesame polarity are oriented to face each other.

The rod 4 passes through the coil holder 15. This arrangement causes thepermanent magnets 6 a to face the coils 7 held on the inner surface ofthe coil holder 15. When inserted into the annular space 21 formedbetween the inner tube 3 and the rod 4, the coil holder 15 causes thepermanent magnets 6 a to face the coils 7. The permanent magnets 6 aretained in the rod 4 produce magnetic fields that act on the coils 7.The permanent magnets 6 a together constitute the magnetic fieldgenerator 6 of the linear actuator 1.

If the cylinder portion 4 a is made of a ferromagnetic material,magnetic flux may concentrate within the cylinder portion 4 a,potentially causing a risk of influencing magnetic flux in an outerperipheral area of the permanent magnets 6 a. Thus, the cylinder portion4 a is made of a nonmagnetic material. Although the six permanentmagnets 6 a are provided in the rod 4, the number of the permanentmagnets 6 a may be determined depending on a required driving force ofthe linear actuator 1. In other words, the number of the permanentmagnets 6 a may be two or more.

In this embodiment, the rod 4 is constructed to have a hollow internalspace for accommodating the permanent magnets 6 a. However, the rod 4 isnot limited to this structure but may be constructed into a columnarshape and the ring-shaped permanent magnets 6 a may be mounted on anouter peripheral surface of the rod 4. In a case where the permanentmagnets 6 a are ring-shaped, the permanent magnets 6 a may be formedsuch that the N and S poles are located along the axial direction or thepermanent magnets 6 a may be magnetized such that the permanent magnets6 a are polarized along inward and outward directions. That is to say,it is only required that the permanent magnets 6 a be configured suchthat the N and S poles are alternately disposed along the axialdirection of the rod 4.

It should be noted that, in this embodiment, a disk-shaped yoke 18 isplaced between one permanent magnet 6 a and an adjacent permanentmagnet. The yokes 18 make it possible to efficiently produce themagnetic fields in an outer peripheral area of the rod 4.

The rod guide 5 fixed to the plug 4 c provided at the other end of therod 4 is formed into a disklike shape. Mounted on an outer peripheralsurface of the rod guide 5 is a dynamic seal 19 that slides along aninner surface of the guide sliding part 11 a of the base portion 11 ofthe outer tube 2. The rod guide 5 is inserted in the guide sliding part11 a of the outer tube 2 with the dynamic seal 19 placed in between. Therod 4 is guided by the rod guide 5 so that an axis of the rod 4 wouldnot swing with respect to the outer tube 2.

The plug 4 b provided at the proximal end of the rod 4 is joined to thebottom portion 3 a of the inner tube 3. The inner tube 3 is positionedalong radial directions within the outer tube 2 by the bearing 13 andthe bearing 17 so that an axis of the inner tube 3 would not swing withrespect to the outer tube 2. Thus, the rod 4 is positioned along theradial directions relative to the outer tube 2 and the inner tube 3 sothat an axis of the rod 4 would not swing with respect to both.

Also, the coils 7 are retained by the outer tube 2 and positioned alongthe radial directions. Thus, the coils 7 and the magnetic fieldgenerator 6 are configured so that both axes do not swing with respectto each other. Therefore, the linear actuator 1 can perform smoothextending and contracting actions.

A gap between the outer tube 2 and the inner tube 3 is sealed by thedynamic seal 14. Also, a gap between the outer tube 2 and the rod guide5 is sealed by the dynamic seal 19. Thus, an internal space of thelinear actuator 1 is kept in a sealed state by the dynamic seal 14 andthe dynamic seal 19.

Referring to FIG. 2(A), in a contracting stroke in which the inner tube3 penetrates the outer tube 2, the linear actuator 1 can continue thestroke until an extreme end of the bearing fitting part 3 c of the innertube 3 comes into contact with the flange portion 15 b of the coilholder 15. The flange portion 15 b thus functions as a stopper forrestricting the stroke of the linear actuator 1.

Referring to FIG. 2(B), in an extending stroke in which the inner tube 3moves backward from the outer tube 2, the linear actuator 1 can continuethe stroke until the bearing fitting part 3 c of the inner tube 3 andthe bearing fitting part 12 b of the outer tube 2 come into contact witheach other.

The extending stroke when the linear actuator 1 is most extended may berestricted by causing the rod guide 5 and the flange portion 15 b tocome into contact with each other. Also, there may be provided a cushionor the like between elements which comes into contact with each otherwhen the stroke is limited to alleviate an impact caused by contactaction produced at the time of restricting the stroke. The rod guide 5slides along the inner surface of the guide sliding part 11 a of theouter tube 2 within a predetermined slide range as a result of theextending or contracting action of the linear actuator 1.

As shown in FIG. 1, the air gap 20 between the outer tube 2 and theinner tube 3 is connected to the internal space of the inner tube 3 viathe conducting hole 3 b. Also, the air gap 20 is connected to theannular space 21 between the inner tube 3 and the rod 4, a space 22formed between the outer tube 2 and the coil holder 15, and a space 23formed by the outer tube 2 and the rod guide 5.

This means that the air gap 20, the annular space 21, the space 22 andthe space 23 formed within the linear actuator 1 are connected to oneanother. The internal volumetric capacity of each of the air gap 20, theannular space 21, the space 22 and the space 23 varies with the strokeof the linear actuator 1.

Here, the cross-sectional area of an outer periphery of the inner tube 3that slides along the outer tube 2 is made equal to the cross-sectionalarea of an inner periphery of the outer tube 2 that slides along the rodguide 5. The cross-sectional area of the outer periphery of the innertube 3 represents a cross sectional area of the inner tube 3, which isperpendicular to a central axis of the inner tube 3. The cross-sectionalarea of the inner periphery of the outer tube 2 represents across-sectional area of an inner space of the outer tube 2, which isperpendicular to a central axis of the outer tube 2.

It should be noted that, although the slidable range in which the rodguide 5 can slide is formed over the entire length of the guide slidingpart 11 a so as not to create an ineffectual portion in the entirelength of the outer tube 2 in the case of this embodiment, there may beprovided a range in which the rod guide 5 does not slide when the linearactuator 1 makes a maximum stroke. In this case, the inner periphery ofthe outer tube 2 within the range in which the rod guide 5 does notslide may not necessarily be formed to have the same cross-sectionalarea as the outer periphery of the inner tube 3.

When the rod 4 makes a stroke, shifting from a state in which the linearactuator 1 is most contracted as shown in FIG. 2(A) to a state in whichthe linear actuator 1 is most extended as shown in FIG. 2(B), thevolumetric capacity of the air gap 20 and that of the space 23 graduallydecrease as a result of the stroke while volumetric capacities of theannular space 21 and the space 22 increase by the same amount.

Specifically, when the rod 4 extends from the contracted state, thevolumetric capacity of the air gap 20 decreases. The volumetric capacityof the air gap 20 decreases by an amount 31 of reduction shown in FIG.2(A). This amount 31 of reduction is equal to the volume of an amount 33of increase among the entire amount of increase by which the volumetriccapacity of the space 22 is increased as shown in FIG. 2(B).

Also, when the rod 4 extends from the contracted state, the volumetriccapacity of the space 23 decreases. The volumetric capacity of the space23 decreases by an amount 32 of reduction shown in FIG. 2(A). Thisamount 32 of reduction is equal to the total volume of an amount 34 ofincrease in the volumetric capacity of the annular space 21 and anamount 35 of increase among the entire amount of increase by which thevolumetric capacity of the space 22 is increased as shown in FIG. 2(B).

The cross-sectional area of the outer periphery of the inner tube 3 ismade equal to the cross-sectional area of the inner periphery of theouter tube 2 as mentioned above. Consequently, when the rod 4 of thelinear actuator 1 makes a stroke in a direction to cause the volumetriccapacities of the air gap 20 and the space 23 to decrease, thevolumetric capacities of the annular space 21 and the space 22 increaseby the same amount of reduction in the volumetric capacities of the airgap 20 and the space 23. When the rod 4 makes a stroke in a direction tocause the volumetric capacities of the annular space 21 and the space 22to decrease, on the other hand, the volumetric capacities of the air gap20 and the space 23 increase by the same amount of reduction in thevolumetric capacities of the annular space 21 and the space 22.Therefore, even when the rod 4 makes a stroke in the linear actuator 1,the internal volumetric capacity of the linear actuator 1 does not varyirrespective of a stroke direction and amount.

Since the internal volumetric capacity does not vary even when thelinear actuator 1 extends or contracts as discussed above, an internalpressure remains unchanged. It is therefore possible to reliably preventforeign matter from being sucked into the internal space of the linearactuator 1.

According to this embodiment, as the outer peripheral surface of theinner tube 3 along which the outer tube 2 slides and the inner surfaceof the outer tube 2 along which the rod guide 5 slides have the samecross-sectional area, both are formed into a circular shape having thesame diameter. However, the shape of the cross-section may be one thatis different from a circular shape as long as the same cross-sectionalarea is maintained.

The annular space 21 and the space 23 are connected to each other via aring-shaped gap formed between the rod 4 and the coil holder 15. It isadvantageous in terms of increasing the thrust of the linear actuator 1if this ring-shaped gap is made as small as possible. If this gap ismade small, however, the gap will present resistance to a flow of gasbetween the annular space 21 and the space 23, potentially causing arisk of generating a damping force that acts against the stroke of therod 4.

To cope with this problem, according to this embodiment, the conductinghole 15 c having a cross-sectional area that will not impede the flow ofgas is formed through the flange portion 15 b of the coil holder 15 tointerconnect the annular space 21 and the space 23. As a result, it ispossible to prevent the generation of the damping force that actsagainst the stroke of the rod 4.

Consequently, the provision of the conducting hole 15 c allows areduction in the gap between the rod 4 and the coil holder 15 and,therefore, it is possible to suppress the generation of the dampingforce and increase a maximum thrust of the linear actuator 1.

The linear actuator 1 according to this embodiment is so structured asto bear the bending moment caused by the lateral force exerted on thelinear actuator 1 by the outer tube 2 and the inner tube 3. Accordingly,a situation in which an excessive bending moment acts on the rod 4 canbe avoided, thereby preventing the permanent magnets 6 a held by the rod4 from undergoing an excessive stress.

The coils 7 are held by the cylindrical coil holder 15 which is fixed tothe outer tube 2 via the flange portion 15 b. Accordingly, a situationin which an excessive bending moment acts on the coils 7 can be avoided,thereby preventing the coils 7 from coming off the coil holder 15 orundergoing a breakage.

The above-described embodiment produces the following advantages.

According to this embodiment, the outer peripheral surface of the innertube 3 along which the outer tube 2 slides and the inner surface of theouter tube 2 along which the rod guide 5 slides are formed to have thesame cross-sectional area. For this reason, even when the rod 4 makes astroke in the linear actuator 1, the internal volumetric capacity of thelinear actuator 1 does not vary. Thus, the internal pressure of thelinear actuator 1 does not vary. Therefore, it is possible to preventintrusion of foreign matter into the internal space of the linearactuator 1.

Although the invention has been described above with reference tocertain embodiments, the invention is not limited to the embodimentsdescribed above. Modifications and variations of the embodimentsdescribed above will occur to those skilled in the art, within the scopeof the claims.

The contents of application No. 2010-210453, with a filing date of Sep.21, 2010 in Japan, are hereby incorporated by reference.

The embodiments of this invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A linear actuatorcomprising an outer tube and an inner tube inserted in the outer tube,the linear actuator generating a thrust for relatively displacing theouter tube and the inner tube along an axial direction; furthercomprising: a rod erected at a central axis part of the inner tube andforming an annular space between the rod and the inner tube; a rod guideprovided at an extreme end of the rod in sliding contact with an innersurface of the outer tube; a plurality of permanent magnets arrangedalong the axial direction in the rod; and a plurality of coils retainedby the outer tube so as to face the permanent magnets; wherein across-sectional area of an outer periphery of the inner tube along whichthe outer tube slides and a cross-sectional area of an inner peripheryof the outer tube along which the rod guide slides are set to be equal.2. The linear actuator as defined in claim 1, wherein the outerperiphery of the inner tube along which the outer tube slides is formedinto a circular shape and the inner periphery of the outer tube alongwhich the rod guide slides is formed into a circular shape having adiameter that is identical to a diameter of the outer periphery of theinner tube.
 3. The linear actuator as defined in claim 1, furthercomprising: a first bearing that is provided in an inner surface of anend portion of the outer tube, in which the inner tube is inserted, andslides along the outer periphery of the inner tube; an annular secondbearing that is provided on an outer peripheral surface of an open endof the inner tube and slides along the inner periphery of the outertube; and a conducting hole interconnecting an annular air gap formedbetween the inner tube and the outer tube and an internal space of theinner tube.
 4. The linear actuator as defined in claim 1, furthercomprising: a cylindrical coil holder which is inserted in the annularspace formed between the rod and the inner tube and retains the coils,wherein the coil holder comprises: a coil holding portion retaining thecoils within an inner surface; and a flange portion joined directly tothe coil holding portion along an axial direction thereof and fixed tothe outer tube.
 5. The linear actuator as defined in claim 4, whereinthe outer tube comprises: a cylindrical base portion having a guidesliding part into which the rod guide is inserted so as to be free toslide and a fitting part which is joined directly to the guide slidingpart along an axial direction thereof with the flange portion fitted inthe fitting part; and a cylindrical inner portion which is screwed intoan end portion of the fitting part, the inner portion having an innersurface along which the inner tube slides; wherein the coil holder isfixed to the outer tube with the flange portion sandwiched between thebase portion and the inner portion.
 6. The linear actuator as defined inclaim 4, wherein the coil holder has a conducting hole passing throughthe flange portion along the axial direction.
 7. The linear actuator asdefined in claim 4, wherein the coil holder comprises a position sensorprovided at the flange portion for detecting a stroke position of therod.